1. Field of the Invention
This invention relates to a redistributed electric field cable and a method of manufacturing same. In one aspect, the invention relates to a corrosion resistant redistributed electric field cable used with devices to analyze geologic formations adjacent a well before completion and a method of manufacturing same.
2. Description of the Related Art
Generally, geologic formations within the earth that contain oil and/or petroleum gas have properties that may be linked with the ability of the formations to contain such products. For example, formations that contain oil or petroleum gas have higher electrical resistivity than those that contain water. Formations generally comprising sandstone or limestone may contain oil or petroleum gas. Formations generally comprising shale, which may also encapsulate oil-bearing formations, may have porosities much greater than that of sandstone or limestone, but, because the grain size of shale is very small, it may be very difficult to remove the oil or gas trapped therein.
Accordingly, it may be desirable to measure various characteristics of the geologic formations adjacent to a well before completion to help in determining the location of an oil- and/or petroleum gas-bearing formation as well as the amount of oil and/or petroleum gas trapped within the formation. Logging tools, which are generally long, pipe-shaped devices, may be lowered into the well to measure such characteristics at different depths along the well. These logging tools may include gamma-ray emitters/receivers, caliper devices, resistivity-measuring devices, neutron emitters/receivers, and the like, which are used to sense characteristics of the formations adjacent the well. A wireline cable connects the logging tool with one or more electrical power sources and data analysis equipment at the earth's surface, as well as providing structural support to the logging tools as they are lowered and raised through the well. Generally, the wireline cable is spooled out of a truck, over a sheave, and down into the well. The wireline cables typically have an outside diameter as small as possible to maximize the cable length on a drum. Other desirable characteristics include high strength to weight rations, high power delivery, high corrosion resistance and good data transmission.
Wireline cables are typically formed from a combination of metallic conductors, insulative material, filler materials, jackets, and metallic armor wires. In the manufacture of cables, it is common to utilize extrusion processing to form an insulating jacket adjacent the conductor, or conductors, of the cable. It is desirable for some applications to form a dielectric cable by using more than one insulative jacket adjacent the conductor(s) to achieve certain dielectric properties. U.S. Pat. No. 6,600,108 (Mydur et al.), incorporated by reference herein, describes cables with two different insulative jackets formed around conductor(s) to provide a cable capable of transmitting larger amounts of power with minimal electrical insulation, by reducing the peak electric field strength induced in the electrical power voltage range. This allows the cable diameter to remain as small as possible. This design may also avoid using the metallic armor as an electrical return conductor, as such configurations may present a hazard to personnel and equipment that inadvertently come into contact with the armor wires during operation of the logging tools. Further, in some applications, dielectric wireline cables are exposed to significant levels of corrosive chemicals, such as hydrogen sulfide.
The presence of corrosive chemicals, such as hydrogen sulfide, in wells or well fluids can cause significant damage to armor wires and metallic conductors. For example, hydrogen sulfide, in the form of a gas or a gas dissolved in liquids, attacks metals by combining with them to form metallic sulfides and atomic hydrogen. The destructive process is principally hydrogen embrittlement, accompanied by chemical attack. Chemical attack may be commonly referred to as sulfide stress cracking. Hydrogen sulfide attacks metals with a wide variation in intensity. High-strength steels used in armor wires, which have high carbon content and are highly cold-worked, are particularly susceptible to damage by hydrogen sulfide. Therefore, metals and special alloys that are very corrosion resistant must be used as armor wire material. To protect against damage by hydrogen sulfide or other corrosive chemicals, specially modified metallic electrical conductors are typically used. The individual metallic conductors are typically coated with metal, typically nickel, before being insulated. Coated conductors have higher resistance that traditional uncoated conductors thereby limiting the ability to transmit power for a given cable diameter.
Coated metallic conductors are prone to having the coating flake off during the manufacture, handling, and use. Because the conductor and coating metals may have differing coefficients of thermal expansion, the coating can flake off when the wire is exposed to the heat of the extruder. The coating may also flake off as the wire is bent over tensioning pulleys. The coating may also be rubbed off through contact friction at the extruder tip. The coating flakes tend to mix with the insulation layers or jackets thereby causing localized electric field enhancement which may lead to partial discharge activity or even a reduction in dielectric strength. This may result in a loss of ability to adequately transmit power.
Thus, a need exists for cables that are capable of transmitting larger amounts of power while maintaining a small cable diameter and remaining corrosion resistant. A cable that can overcome the problems detailed above while transmitting larger amounts of power while maintaining data signal transmission integrity would be highly desirable, and the need is met at least in part by the following invention.